Electromechanical servo control systems as applied to electromechanical positioning and velocity control systems are often adversely affected by mechanical actuator resonances. These resonances generally cannot be damped effectively by conventional servo control systems. Typical servo systems that have mechanical actuator resonances include those used for transducer head positioning in disc drives. These mechanical actuator resonances limit control loop gain of the servo system, reduce bandwidth of the servo system, or both. This causes the controlled element, such as a transducer head, to have excessive settling time after positioning, poor response to disturbances, poor tracking ability, or any combination of these.
Control loop stability problems may also result from these lightly damped structural resonances that are associated with the mechanical actuator. Prior art systems have made use of gain stabilizing filters such as electronic notch filters inserted in the control loop path. These filters are inserted in the forward path of the control loop to filter out the signal information within the band reject frequency range of the notch and therefore help minimize excitation of these actuator resonances by the servo control system itself.
The technique utilizing notch filters described above allows the servo control system to effectively ignore lightly damped structural actuator resonances in that very little control effort may be applied by the servo controller at frequencies where notch filters attenuate the control signal. However, the use of notch filters does nothing to reduce the sensitivity of the servo system to other types of disturbances that excite the mechanical resonances, such as those caused by servo amplifier saturation and distortion, external forces on the carriage such as caused by seek activity, air turbulence, stiction, and so forth. This is because such disturbances are typically generated at points in the control which does not lend themselves to correction when such gain stabilizing filters exist in the control loop. Although the notch filters, coupled in a forward path configuration, will serve to reduce steady state frequency components of the control signal in the bandwidth of the notched structural resonances, they do not necessarily inhibit the excitation of these resonances by these disturbances external to the servo loop.
The exciting effect of power amplifier non-linearity, when due to saturation or cross-over distortion, is generally not eliminated with the notch filtering process, and such forms of disturbance, appearing directly at the input to the mechanical process to be controlled, continues to provide undesirable excitation of the lightly damped structural resonances. Likewise, seek induced excitation and other mechanical disturbances that are due to external excitation other than that of the actuator itself are not reduced by the notch filtering process.
Gain stabilization in the form of low-pass filtering is also used for control loop gain stabilization. In this case, the cutoff frequency of a low-pass filter that is inserted in the control loop is generally lower than the frequencies of any of the lightly damped resonances of the actuator structure that are desired to be attenuated in their effect on the control loop. In this way, the signal components of the control signal are substantially prevented from exciting the lightly damped resonances of the actuator structure. This helps ensure system stability, but it also increases phase shift at frequencies in the vicinity of the servo loop's unity gain crossing, thereby reducing the bandwidth of the servo system. This is true of all gain stabilizing filters, including notch filters. This reduction in bandwidth in turn reduces the ability of the servo system to correct low frequency vibration and tracking performance such as run out and other disturbances that are due to external excitation and non-linearities in positioning operations.
Typical servo control systems sense motion at so called "non-collocated" locations on the actuator or its payload, such as at arm tips on disc drives. "Non-collocated" in this context means that there is substantial flexibility between the means of sensing vibration and the "point of control" for the structure. The "point of control" is the location on the actuator structure where control effort is applied.
The prior art approaches to non-collocated control, as applied to servo systems for transducer head positioning, cannot robustly dampen structural resonances, such as the lightly damped motor actuator resonances, because adequate loop gain cannot be consistently maintained at or near resonant frequencies, especially if there is variability in the magnitude or damping of actuator resonances. This is because the non-uniform phase characteristic of the actuator practically limits the loop bandwidth attainable with closed loop stability, a mandatory requirement. For such systems, the loop phase uncertainty becomes too high to close a stable high gain feedback loop at or near resonant frequencies as required for robust active damping. "Robust" means the control system is relatively insensitive to changes in actuator dynamics, in this case, resonances.